A plurality of electronic apparatuses are often used coupled to each other. In addition, in a personal computer (PC) and the like, a function thereof is to couple a plurality of peripheral devices such as a printer, a display, a scanner, a hard disc drive, and the like thereto. In the electronic apparatuses and the peripheral devices, clock signals thereof, which operate these apparatuses and devices properly, are often different from each other. Therefore, an oscillator provided to a mother board outputs a plurality of clock signals having different frequencies in order that the peripheral devices coupled to the mother board of the electronic apparatus operate properly (see for example, Patent Document JP-A-9-237133). In a case where a plurality of clock signals having different frequencies are output as mentioned above, a clock signal output by one oscillator is frequency-transformed by PLL circuit.
A reference clock signal to be input into the peripheral devices coupled to the mother board often includes jitters due to an effect of a wiring of the mother board or noises from the outside.
Further, an input-output interface called a PCI express has become widespread in use recently. The PCI express outputs a 100 MHz reference clock signal obtained by spectrum spreading on a mother board, and apparatuses coupled to the mother board operate at 100 MHz or 125 MHz. Therefore, the PCI express needs to output 100 MHz or 125 MHz clock signals while corresponding to operating frequencies of electronic apparatuses coupled to the mother board. Accordingly, the PCI express needs a PLL circuit which can switch and output 100 MHz or 125 MHz.